/*
 *
 *  * Copyright (C) 2015 Square, Inc.
 *  *
 *  * Licensed under the Apache License, Version 2.0 (the "License");
 *  * you may not use this file except in compliance with the License.
 *  * You may obtain a copy of the License at
 *  *
 *  *      http://www.apache.org/licenses/LICENSE-2.0
 *  *
 *  * Unless required by applicable law or agreed to in writing, software
 *  * distributed under the License is distributed on an "AS IS" BASIS,
 *  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  * See the License for the specific language governing permissions and
 *  * limitations under the License.
 *
 */
package com.lark.oapi.okio;

import java.io.IOException;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.util.Arrays;

import static com.lark.oapi.okio.Util.arrayRangeEquals;
import static com.lark.oapi.okio.Util.checkOffsetAndCount;

/**
 * An immutable byte string composed of segments of byte arrays. This class exists to implement
 * efficient snapshots of buffers. It is implemented as an array of segments, plus a directory in
 * two halves that describes how the segments compose this byte string.
 *
 * <p>The first half of the directory is the cumulative byte count covered by each segment. The
 * element at {@code directory[0]} contains the number of bytes held in {@code segments[0]}; the
 * element at {@code directory[1]} contains the number of bytes held in {@code segments[0] +
 * segments[1]}, and so on. The element at {@code directory[segments.length - 1]} contains the total
 * size of this byte string. The first half of the directory is always monotonically increasing.
 *
 * <p>The second half of the directory is the offset in {@code segments} of the first content byte.
 * Bytes preceding this offset are unused, as are bytes beyond the segment's effective size.
 *
 * <p>Suppose we have a byte string, {@code [A, B, C, D, E, F, G, H, I, J, K, L, M]} that is stored
 * across three byte arrays: {@code [x, x, x, x, A, B, C, D, E, x, x, x]}, {@code [x, F, G]}, and
 * {@code [H, I, J, K, L, M, x, x, x, x, x, x]}. The three byte arrays would be stored in {@code
 * segments} in order. Since the arrays contribute 5, 2, and 6 elements respectively, the directory
 * starts with {@code [5, 7, 13} to hold the cumulative total at each position. Since the offsets
 * into the arrays are 4, 1, and 0 respectively, the directory ends with {@code 4, 1, 0]}.
 * Concatenating these two halves, the complete directory is {@code [5, 7, 13, 4, 1, 0]}.
 *
 * <p>This structure is chosen so that the segment holding a particular offset can be found by
 * binary search. We use one array rather than two for the directory as a micro-optimization.
 */
final class SegmentedByteString extends ByteString {

    final transient byte[][] segments;
    final transient int[] directory;

    SegmentedByteString(Buffer buffer, int byteCount) {
        super(null);
        checkOffsetAndCount(buffer.size, 0, byteCount);

        // Walk through the buffer to count how many segments we'll need.
        int offset = 0;
        int segmentCount = 0;
        for (Segment s = buffer.head; offset < byteCount; s = s.next) {
            if (s.limit == s.pos) {
                throw new AssertionError("s.limit == s.pos"); // Empty segment. This should not happen!
            }
            offset += s.limit - s.pos;
            segmentCount++;
        }

        // Walk through the buffer again to assign segments and build the directory.
        this.segments = new byte[segmentCount][];
        this.directory = new int[segmentCount * 2];
        offset = 0;
        segmentCount = 0;
        for (Segment s = buffer.head; offset < byteCount; s = s.next) {
            segments[segmentCount] = s.data;
            offset += s.limit - s.pos;
            if (offset > byteCount) {
                offset = byteCount; // Despite sharing more bytes, only report having up to byteCount.
            }
            directory[segmentCount] = offset;
            directory[segmentCount + segments.length] = s.pos;
            s.shared = true;
            segmentCount++;
        }
    }

    @Override
    public String utf8() {
        return toByteString().utf8();
    }

    @Override
    public String string(Charset charset) {
        return toByteString().string(charset);
    }

    @Override
    public String base64() {
        return toByteString().base64();
    }

    @Override
    public String hex() {
        return toByteString().hex();
    }

    @Override
    public ByteString toAsciiLowercase() {
        return toByteString().toAsciiLowercase();
    }

    @Override
    public ByteString toAsciiUppercase() {
        return toByteString().toAsciiUppercase();
    }

    @Override
    public ByteString md5() {
        return toByteString().md5();
    }

    @Override
    public ByteString sha1() {
        return toByteString().sha1();
    }

    @Override
    public ByteString sha256() {
        return toByteString().sha256();
    }

    @Override
    public ByteString hmacSha1(ByteString key) {
        return toByteString().hmacSha1(key);
    }

    @Override
    public ByteString hmacSha256(ByteString key) {
        return toByteString().hmacSha256(key);
    }

    @Override
    public String base64Url() {
        return toByteString().base64Url();
    }

    @Override
    public ByteString substring(int beginIndex) {
        return toByteString().substring(beginIndex);
    }

    @Override
    public ByteString substring(int beginIndex, int endIndex) {
        return toByteString().substring(beginIndex, endIndex);
    }

    @Override
    public byte getByte(int pos) {
        checkOffsetAndCount(directory[segments.length - 1], pos, 1);
        int segment = segment(pos);
        int segmentOffset = segment == 0 ? 0 : directory[segment - 1];
        int segmentPos = directory[segment + segments.length];
        return segments[segment][pos - segmentOffset + segmentPos];
    }

    /**
     * Returns the index of the segment that contains the byte at {@code pos}.
     */
    private int segment(int pos) {
        // Search for (pos + 1) instead of (pos) because the directory holds sizes, not indexes.
        int i = Arrays.binarySearch(directory, 0, segments.length, pos + 1);
        return i >= 0 ? i : ~i; // If i is negative, bitflip to get the insert position.
    }

    @Override
    public int size() {
        return directory[segments.length - 1];
    }

    @Override
    public byte[] toByteArray() {
        byte[] result = new byte[directory[segments.length - 1]];
        int segmentOffset = 0;
        for (int s = 0, segmentCount = segments.length; s < segmentCount; s++) {
            int segmentPos = directory[segmentCount + s];
            int nextSegmentOffset = directory[s];
            System.arraycopy(segments[s], segmentPos, result, segmentOffset,
                    nextSegmentOffset - segmentOffset);
            segmentOffset = nextSegmentOffset;
        }
        return result;
    }

    @Override
    public ByteBuffer asByteBuffer() {
        return ByteBuffer.wrap(toByteArray()).asReadOnlyBuffer();
    }

    @Override
    public void write(OutputStream out) throws IOException {
        if (out == null) {
            throw new IllegalArgumentException("out == null");
        }
        int segmentOffset = 0;
        for (int s = 0, segmentCount = segments.length; s < segmentCount; s++) {
            int segmentPos = directory[segmentCount + s];
            int nextSegmentOffset = directory[s];
            out.write(segments[s], segmentPos, nextSegmentOffset - segmentOffset);
            segmentOffset = nextSegmentOffset;
        }
    }

    @Override
    void write(Buffer buffer) {
        int segmentOffset = 0;
        for (int s = 0, segmentCount = segments.length; s < segmentCount; s++) {
            int segmentPos = directory[segmentCount + s];
            int nextSegmentOffset = directory[s];
            Segment segment = new Segment(segments[s], segmentPos,
                    segmentPos + nextSegmentOffset - segmentOffset, true, false);
            if (buffer.head == null) {
                buffer.head = segment.next = segment.prev = segment;
            } else {
                buffer.head.prev.push(segment);
            }
            segmentOffset = nextSegmentOffset;
        }
        buffer.size += segmentOffset;
    }

    @Override
    public boolean rangeEquals(
            int offset, ByteString other, int otherOffset, int byteCount) {
        if (offset < 0 || offset > size() - byteCount) {
            return false;
        }
        // Go segment-by-segment through this, passing arrays to other's rangeEquals().
        for (int s = segment(offset); byteCount > 0; s++) {
            int segmentOffset = s == 0 ? 0 : directory[s - 1];
            int segmentSize = directory[s] - segmentOffset;
            int stepSize = Math.min(byteCount, segmentOffset + segmentSize - offset);
            int segmentPos = directory[segments.length + s];
            int arrayOffset = offset - segmentOffset + segmentPos;
            if (!other.rangeEquals(otherOffset, segments[s], arrayOffset, stepSize)) {
                return false;
            }
            offset += stepSize;
            otherOffset += stepSize;
            byteCount -= stepSize;
        }
        return true;
    }

    @Override
    public boolean rangeEquals(int offset, byte[] other, int otherOffset, int byteCount) {
        if (offset < 0 || offset > size() - byteCount
                || otherOffset < 0 || otherOffset > other.length - byteCount) {
            return false;
        }
        // Go segment-by-segment through this, comparing ranges of arrays.
        for (int s = segment(offset); byteCount > 0; s++) {
            int segmentOffset = s == 0 ? 0 : directory[s - 1];
            int segmentSize = directory[s] - segmentOffset;
            int stepSize = Math.min(byteCount, segmentOffset + segmentSize - offset);
            int segmentPos = directory[segments.length + s];
            int arrayOffset = offset - segmentOffset + segmentPos;
            if (!arrayRangeEquals(segments[s], arrayOffset, other, otherOffset, stepSize)) {
                return false;
            }
            offset += stepSize;
            otherOffset += stepSize;
            byteCount -= stepSize;
        }
        return true;
    }

    @Override
    public int indexOf(byte[] other, int fromIndex) {
        return toByteString().indexOf(other, fromIndex);
    }

    @Override
    public int lastIndexOf(byte[] other, int fromIndex) {
        return toByteString().lastIndexOf(other, fromIndex);
    }

    /**
     * Returns a copy as a non-segmented byte string.
     */
    private ByteString toByteString() {
        return new ByteString(toByteArray());
    }

    @Override
    byte[] internalArray() {
        return toByteArray();
    }

    @Override
    public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        return o instanceof ByteString
                && ((ByteString) o).size() == size()
                && rangeEquals(0, ((ByteString) o), 0, size());
    }

    @Override
    public int hashCode() {
        int result = hashCode;
        if (result != 0) {
            return result;
        }

        // Equivalent to Arrays.hashCode(toByteArray()).
        result = 1;
        int segmentOffset = 0;
        for (int s = 0, segmentCount = segments.length; s < segmentCount; s++) {
            byte[] segment = segments[s];
            int segmentPos = directory[segmentCount + s];
            int nextSegmentOffset = directory[s];
            int segmentSize = nextSegmentOffset - segmentOffset;
            for (int i = segmentPos, limit = segmentPos + segmentSize; i < limit; i++) {
                result = (31 * result) + segment[i];
            }
            segmentOffset = nextSegmentOffset;
        }
        return (hashCode = result);
    }

    @Override
    public String toString() {
        return toByteString().toString();
    }

    private Object writeReplace() {
        return toByteString();
    }
}
